Most fluxes and defluxing chemicals used by industry to manufacture electronic circuits, such as military hardware, are either contributing to ozone depletion of the atmosphere or are considered by local environmental agencies, such as the Air Quality Management District in the Los Angeles basin, as environmental pollutants or health hazards. For example, it has been reported that if CFCs (chlorofluorohydrocarbons), which are used in vapor degreasing to clean soldered parts where a rosin flux is employed, are released to the atmosphere, they will remain there as an ozone depletant for nearly 100 years. Other chemicals such as rosin fluxes, alcohols, and the like pose health risks and disposal problems for industry.
Water-soluble fluxes could be the simple solution for this major problem. However, most water-soluble fluxes are formulated with harsh activators, such as hydrochloric acid and complex glycols, which create major cleaning and residue problems for the printed circuit board and the electrical circuitry thereon and which tend to severely corrode the metal being soldered or contaminate the dielectric, causing a tendency for electromigration. Other water-soluble fluxes are formulated with iso-propanol and/or plasticizers, which create disposal and health problems.
Various attempts have been made to improve the quality of solder fluxes. In many instances, carboxylic acids of various types have been added.
For example, U.S. Pat. No. 3,020,635, issued to H. F. Redgrift, discloses soldering methods. Specifically, Redgrift discloses the use of a solution of lactic acid as a flux, which may be a 15 to 25% solution of lactic acid in water. Alternatively, a 40% solution of ortho-phosphoric acid in water may be used. However, the present applicant has found that a flux containing 40 wt. % lactic acid
exhibited incomplete close-up of 60/40 tin/lead solder rings (3/8 inch diameter) placed on copper panels and also showed evidence of copper corrosion. Further, hand-soldered connections to 14-lead dual inline packages (DIPs) onto printed wiring boards (PWBs) exhibited solder discontinuities (voids), rough porous solder quality, and excess stress lines within the solder fillets. Finally, during wave soldering processes, used to solder DIPs onto PWBs, the lactic acid flux smoked profusely when the board entered the solder zone. The lactic acid fluxed portion of the PWB exhibited solder shorts, solder horns, solder webbing, and solder bridges too numerous to count.
U.S. Pat. No. 4,809,901 issued to T. G. Gen et al discloses soldering methods and devices. The disclosure includes use of a flux composition which is capable of undergoing a visible color change at a critical temperature. The flux is a commercially available rosin-based flux comprising 37% of a mixture of rosin isomers in 63% iso-propyl alcohol (Col. 6, lines 12-16). To this flux may be added
"dispersing aids such as water, alcohol or a subfactant [sic: surfactant] or emulsifier, buffers, such as an acid or salt to maintain the desired pH, for example, oxalic acid, lactic acid, maleic acid, tartaric acid, fumaric acid or citric acid, stabilizers and the like".
However, rosin-based fluxes require CFCs in a vapor-degreasing process to clean soldered parts. As noted above, the use of CFCs has been found to contribute to the depletion of the ozone layer, and hence efforts are underway to drastically reduce or even eliminate the use of CFCs in processing. Further, rosin-based fluxes pose health risks and disposal problems for industry.
Additional references disclosing the use of carboxylic acids as components of fluxes include U.S. Pat. No. 4,568,395, issued to A. R. Nabhani, U.S. Pat. No. 4,460,427, issued to D. M. Haney et al, and U.S. Pat. No. 3,091,029, issued to C. L. Davis et al. These references typically contain additional components that are harmful to the environment.
Japanese Unexamined Patent Application No. 62-6796 discloses the use of carboxylic acid-based fluxes. However, the fluxes require at least two kinds of carboxylic acids, such as oxysuccinic acid, tartaric acid, citric acid, glycolic acid, lactic acid, hydroacrylic acid, oxybutylic acid, glyceric acid, and tartaronic acid, in a solvent such as glycol ethers (e.g., methyl cellusolve, ethyl cellusolve, and butyl cellusolve), alcohols (e.g., methyl alcohol, ethyl alcohol, and iso-propyl alcohol), and water, alone or in combination. Again, many of these components are harmful to the environment.
What is desired is a simple, non-toxic, non-corrosive liquid soldering flux that produces solder joints of high metallic luster and excellent quality, regardless of the method of soldering (hand, wave, drag, etc.).